Transient growth and coupling of vortex and wave modes in self-gravitating gaseous discs

TL;DR

This paper investigates how self-gravity enhances transient growth of vortex modes and reveals asymmetric coupling between vortex and wave modes in self-gravitating astrophysical discs, highlighting the vortex mode's significance.

Contribution

It demonstrates that self-gravity significantly amplifies vortex mode transient growth and uncovers asymmetric mode coupling in self-gravitating discs.

Findings

Self-gravity increases vortex mode transient growth by several orders of magnitude.

Vortex modes can excite wave modes, but not vice versa.

Vortex modes play a crucial role in spiral density waves and shocks.

Abstract

Flow nonnormality induced linear transient phenomena in thin self-gravitating astrophysical discs are studied in the shearing sheet approximation. The considered system includes two modes of perturbations: vortex and (spiral density) wave. It is shown that self-gravity considerably alters the vortex mode dynamics -- its transient (swing) growth may be several orders of magnitude stronger than in the non-self-gravitating case and 2-3 times larger than the transient growth of the wave mode. Based on this finding, we comment on the role of vortex mode perturbations in a gravitoturbulent state. Also described is the linear coupling of the perturbation modes, caused by the differential character of disc rotation. The coupling is asymmetric -- vortex mode perturbations are able to excite wave mode ones, but not vice versa. This asymmetric coupling lends additional significance to the vortex mode as a participant in spiral density waves and shocks manifestations in astrophysical discs.